1. Field of the Invention
This invention relates generally to optical systems used in semiconductor manufacturing.
2. Related Art
Semiconductor devices are typically manufactured using various photolithographic techniques. The circuitry used in a semiconductor chip is projected from a reticle onto a wafer. This projection is often accomplished with the use of optical systems. The design of these optical systems is often complex, and it is difficult to obtain the desired resolution necessary for reproducing the ever-decreasing size of components being placed on a semiconductor chip. Therefore, there has been much effort expended to develop an optical reduction system capable of reproducing very fine component features, less than 0.25 microns. The need to develop an optical system capable of reproducing very fine component features requires the improvement of system performance.
A conventional optical system is disclosed in U.S. Pat. No. 5,537,260 entitled xe2x80x9cCatadioptric Optical Reduction System with High Numerical Aperturexe2x80x9d issued Jul. 16, 1996 to Williamson, which is incorporated by reference herein in its entirety. This reference describes an optical reduction system having a numerical aperture of 0.35. Another optical system is described in U.S. Pat. No. 4,953,960 entitled xe2x80x9cOptical Reduction Systemxe2x80x9d issuing Sep. 4, 1990 to Williamson, which is incorporated by reference herein in its entirety. This reference describes an optical system operating in the range of 248 nanometers and having a numerical aperture of 0.45.
While these optical systems perform adequately for their intended purpose, there is an ever increasing need to improve system performance. The present inventor has identified that a need exists for minimizing the effects of reticle birefringence. Further, there is a need for an optical system having low linewidth critical dimension (CD) control errors capable of acceptable system performance.
The introduction of compensation to control illumination polarization limits the performance impact of reticle birefringence on polarization sensitive projection optics including catadioptric projection optic. When the reticle substrate exhibits birefringence there will be a change in the polarization of the light projected through the optical system.
This change alters the performance of the entire system. Characteristics such as reflectivity, insertion loss, and beam splitter ratios will be different for different polarizations. This leads to dose errors at the wafer. Dose errors contribute toward linewidth CD control errors.
Furthermore, even for a perfectly preferred polarization, dose errors can occur from reticle birefringence. This effect will be relatively small, but if the reticle substrate exhibits birefringence and the input light has a small error in it, then the dose errors will be much greater. The present invention minimizes the effect of reticle birefringence by optimizing the illumination by making very small changes to the illumination polarization based on the input light. Dose error is thereby minimized. This minimization results in a reduction in linewidth CD control error.
In one embodiment, a catadioptric optical reduction system has a variable compensation for reticle retardation before the long conjugate end. The variable compensation component(s) before the reticle provides adjustable elliptically polarized light at or near the reticle. The variable compensation components can be variable wave plates, reflective or transmissive thin film polarizers, a Berek""s compensator and/or a Soleil-Babinet compensator.
In some applications, the reticle has small amounts, much less than a wavelength, of birefringence. In such applications, the birefringence varies over the reticle. This varying birefringence alters the desired polarization state introducing dose errors and associated CD linewidth variation that are a function of position.
The polarization compensator allows for optimization of the illumination polarization state to minimize the dose errors. These small changes adjust the polarization purity to a better level of perfection in comparison to the few percent polarization purity traditionally required in optical systems.
The polarization state purity is adjusted by making small changes to the actual polarization ellipticity. In general, the compensator can be located at anywhere in the illumination system to change the polarization state at the reticle.
However, if there are any strong polarizers (for example, polarization analyzers), then the compensator should be located in the reticle side of the polarizers.
The catadioptric optical reduction system provides a relatively high numerical aperture of 0.7 capable of patterning features smaller than 0.25 microns over a 26 mmxc3x975 mm field. The optical reduction system is thereby well adapted for photolithographic applications and tools, such as a step and scan microlithographic exposure tool as used in semiconductor manufacturing. Several other embodiments combine elements of different refracting power to widen the spectral bandwidth which can be achieved.
In another embodiment, the present invention is a catadioptric reduction system having, from the object or long conjugate end to the reduced image or short conjugate end, a polarization compensator, a reticle, a first lens group, a second lens group, a beamsplitter cube, a concentric concave mirror, and a third lens group. The concave mirror operates near unit magnification. This reduces the aberrations introduced by the mirror and the diameter of radiation entering the beamsplitter cube. The first and second lens groups before the concave mirror provide enough power to image the entrance pupil at infinity at the aperture stop at or near the concave mirror. The third lens group after the concave mirror provides a substantial portion of the reduction from object to image of the optical system, as well as projecting the aperture stop to an infinite exit pupil. High-order aberrations are reduced by using an aspheric concave mirror.